Graded field cathode ray tube

ABSTRACT

A cathode ray tube having a screened flat faceplate and an electron gun oriented off-normal thereto, in which means are provided to produce in the vicinity of the screen an electrostatic field shaped so as to sharpen electron beam focus over the entire screen and reduce raster distortion thereover. Discrete sections of the backplate portion of the tube are coated with conductive material. Operating potentials are supplied to each such conductive section and to the screen to create electrostatic fields in the regions between the screen and each conductive section. The shape of the conductive sections and the operating potentials are selected so that the forces exerted on the electron beam when deflected toward the portion of the screen remote from the electron gun bend the beam toward the screen. This bending causes the beam to impinge on the screen along a path more nearly perpendicular to the screen at the point of impingement of the beam than the path which the beam would follow in the absence of such bending. As a result the area of the region of intersection of beam and screen is reduced, sharpening the focus. By additional appropriate shaping of the electrostatic fields, raster distortion also is reduced.

tates Iadicicco et a1.

atent [191 [54] GRADED FIELD CATHODE RAY TUBE [75] Inventors: Robert A. Iadicicco, Philadelphia;

'Iaul G. Wolfe, Oreland, both of Pa.

[73] Assignee: Philco-Ford Corporation, Philadelphia, Pa.

[22] Filed: June 15, 1970 [21] Appl. No.: 46,020

UNITED STATES PATENTS 3,377,500 4/1968 Burns ..313/78 X 3,435,269 3/1969 Shanafelt et al.. ....313/77 X 3,461,333 8/1969 Havn ..3l3/77 3,005,124 10/1961 Aiken ..313/92 2,928,014 3/1960 Aiken et al ..313/78 FOREIGN PATENTS OR APPLICATIONS 903,587 8/1962 Great Britain OTHER PUBLICATIONS Ramberg, Electron-Optical Properties of a Flat Television Picture Tube, Proceedings of the IRE, December, 1960, pp. l952l960.

[ 51 May 1,1973

Primary ExaminerCarl D. Quarforth Assistant Examiner-E. E. Lehmann Att0rneyI-Ierbert Epstein [57] ABSTRACT A cathode ray tube having a screened flat faceplate and an electron gun oriented off-normal thereto, in which means are provided to produce in the vicinity of the screen an electrostatic field shaped so as to sharpen electron beam focus over the entire screen and reduce raster distortion thereover.

Discrete sections of the backplate portion of the tube are coated with conductive material. Operating potentials are supplied to each such conductive section and to the screen to create electrostatic fields in the regions between the screen and each conductive section. The shape of the conductive sections and the operating potentials are selected so that the forces exerted on the electron beam when deflected toward the portion of the screen remote from the electron gun bend the beam toward the screen. This bending causes the beam to impinge on the screen along a path more nearly perpendicular to the screen at the point of impingement of the beam than the path which the beam would follow in'the absence of such bending. As a result the area of the region of intersection of beam and screen is reduced, sharpening the focus. By additional appropriate shaping of the electrostatic fields, raster distortion also is reduced.

8 Claims, 5 Drawing Figures GRADED FIELD CATHOIDE RAY TUBE BACKGROUND OF THE INVENTION Cathode ray tubes in which the screened viewing faceplate is substantially flat and the beam deflection system of the tube is positioned so that a line from the center of deflection of the tube to the center of the faceplate of the tube makes an acute angle with the faceplate are hereinafter referred to as flat cathode ray tubes" or flat tubes. Flat tubes are advantageous because they have flat viewing screens and occupy less space in a direction perpendicular to the faceplate than do cathode ray tubes in which the line from the center of deflection to the center of the faceplate is perpendicular to the faceplate. The latter tubes are referred to hereinafter as conventional tubes. However, flat tubes have beam-focusing and raster distortion problems not found in conventional cathode ray tubes.

Because of the aforedescribed angular relationship in a flat tube, an electron beam of circular cross section produces, wherever it impinges on the faceplate, an elliptical spot elongated within a narrow range of directions. By contrast, in a conventional cathode-ray tube, an electron beam of circular cross section produces a circular spot of light when perpendicular to the faceplate of the tube and, when not perpendicular, produces a spot elongated in a direction which changes as the point of impingement of the beam is changed. In both kinds of tubes it is desirable that a sharply-focused circular spot of light be produced wherever the beam impinges the viewing screen. To this end, focus correction systems for altering the cross sectional shape of the beam have been proposed for conventional tubes. However, since the way in which spots elongate in flat tubes is considerably different from the way in which spots elongate in conventional tubes, the focus correction systems applicable to conventional tubes are useless in flat tubes.

One system which has been proposed for improving focusing in flat tubes is the use of an electron gun employing elliptical apertures oriented to produce a beam of elliptical cross-section whose major axis is transverse to the direction of elongation. That system is described in copending United States Patent application Ser. No. 772,232, filed Oct. 31, 1968, by R. C. Jones entitled Electron Gun Structure for Flat Cathode Ray Tube, issued on May 18, 1971 as US. Pat. No. 3,579,010, and assigned to the assignee of the present application. Although the latter system considerably improves beam focusing in flat tubes, it does not produce perfect correction of spot ellipticity over the entire screen of the flat tube.

In addition flat tubes present raster distortion effects not easily corrected by methods heretofore used in conventional tubes. The raster is the predetermined pattern of scanning lines to be traced on the viewing screen by the electron beam as it is deflected both vertically and horizontally to produce a picture on the screen. This desired pattern is bounded by straight-line sides. Presence of imperfections in the deflection apparatus and in the deflection signal causes the beam to trace a pattern deviating from the raster. Such deviant pattern is referred to herein as a distorted raster." For example, two common types of raster distortion are pincushion distortion, wherein the sides of the raster are concave, and barrel distortion, wherein the sides of the raster are convex. The type of distortion which occurs depends on the position and characteristics of the particular deflection apparatus being used.

In magnetic deflection systems used in conventional tubes, raster distortion is minimized by shaping of the magnetic field employed for deflection, e.g. by appropriate placement of specially shaped permanent magnets, or by appropriate distribution of the windings of the deflection yoke, or by shaping of deflection waveform or by combinations of these expedients. However the lack of symmetry of the tube structure about the deflection center in a flat tube makes such techniques more difficult to use in connection therewith.

Accordingly, an object of this invention is to provide an improved flat cathode ray tube.

Another object is to provide a flat cathode ray tube in which the electron beam produces a substantially circular spot wherever it impinges on the screen of the tube.

Another object is to provide a flat cathode ray tube wherein the electron beam is bent so as to strike the screen of the tube along a path oriented closer to the perpendicular to the screen at the point of impingement than the path which the beam would follow in the absence of such bending.

Another object is to provide a flat cathode ray tube with reduced raster distortion.

DRAWING FIG. 1 of the drawing is a plan view of a flat cathode ray tube.

FIG. 2 is a sectional view taken along the line 22 in FIG. 1.

FIG. 3 is a view of the interior surface of the backplate of the tube of FIG. 1 and the conductive sections applied to that backplate.

FIGS. 4A and 4B illustrate alternative embodiments of the interior surface of the backplate and the conduc tive sections applied thereto.

DESCRIPTION OF THE APPARATUS FIGS. 1 and 2 show a flat cathode ray tube in accordance with the present invention. The tube includes an envelope 4 comprising a side panel 3, a faceplate 5, a backplate 14, and a neck section 10. A funnel section 6 connects the neck section 10 to the faceplate S and to the backplate 14 at an angle such that a line 34 from the center of deflection 3d of the tube to the center of faceplate 5 forms an acute angle 36 with faceplate 5. Anelectron gun 28 in neck section 10 is adapted to project an electron beam onto a target 32 which, in the embodiment of the invention shown, is a luminescent screen 32 on faceplate 5. Screen 32 may comprise a layer of phosphor material with a film 35 of aluminum thereon. Alternatively, screen 32 may be any other type of electron beam responsive screen such as the type of screen used in a storage tube.

Beam deflection apparatus 8, which surrounds neck section 10, is a deflection yoke which comprises, for example, first and second pairs of coils producing mutually orthogonal magnetic deflection fields in response to deflection currents supplied to the coils. While deflection apparatus 8 is depicted as a magnetic deflection yoke surrounding neck section 10, apparatus 8 alternatively may comprise pairs of opposing electrostatic plates within neck 8, wherein deflection of the electron beam is achieved by applying appropriate potentials to the plates.

Neck 110 is closed at its free end with a stem structure which includes a plurality of lead-in conductors for applying suitable voltages to the electrodes of an electron gun 28. Preferably that electron gun produces an electron beam of elliptical cross-section and has the structure and orientation described in said US. Pat. No. 3,579,010, ofJones.

The interior surface of backplate 114 is coated with conductive materials 18, 22, and 26, for shaping an electrostatic field within the tube. FIG. 3 depicts the interior surface of backplate l4 and the conductive sections applied thereon. The thickness of the sections is exaggerated for clarity. In the embodiment of the invention shown, conductive sections 18, 22, and 26 are of approximately equal dimension and comprise Aquadag, a composition of graphite and water which is painted onto the backplate. Alternatively, the conductive sections may comprise any other type of conductive composition which can withstand the high temperatures normally used in the construction of the tube, e.g. temperatures up to 450C.

Conductive sections 18, 22, 26, are electrically isolated from each other by insulating strips 119 and 23. In the embodiment of the invention shown, Insulating strips 19 and 23 comprise chromium oxide (Cr O which is painted on to the backplate. An additional strip 17 of chromium oxide also is applied at the side of section 13 nearest end 3.

Anode pins 116, 20, and 24, respectively, are connected on one end to conducting sections 18, 22, and 26, respectively, and extend through the backplate. In the embodiment shown, operating potentials which typically may be about 10,000 volts, 12,500 volts, and 15,000 volts, respectively with respect to the potential of the cathode (not shown) of the flat tube, are applied via anode pins 16, 20, and 24 respectively. Series-connected sources 50, 52 and 54 for supplying those potentials are depicted schematically in FIG. 2. In practice, sources 50, 52, 54 taken collectively may comprise a conventional source of high voltage, e.g. the flyback winding of the horizontal deflection transformer of a receiver including the flat picture tube and a rectifier-filter circuit coupled to that winding, and a resistor shunted across the output of the filter and tapped at those locations thereof at which appear the respective operating potentials to be supplied to pins 16, 20 and 24. A conductive coating 31, e.g. of Aquadag, connects aluminum coating 35 on screen 32 to conductive section 26, as well as providing within neck a final acceleration anode for the electron beam, so that the same operating potential which is applied to conductive section 26 is also applied to screen 32.

In practice the glass envelope 4 is assembled by fritsealing together at 56, 58 an upper envelope portion 60 and a lower envelope portion 62. Prior to assembly the phosphor screen 32 and aluminum layer 35 are applied as shown to the interior surface of envelope portion 62, while Aquadag portions 18, 22, 26 and 31, and chromic oxide portions 17, 19, and 23 are applied as shown to the interior surface of envelope portion 60. To connect Aquadag layer 31 to aluminum layer 35, a metal clip, represented in FIG. 4 by element 64, is employed.

FIG. 4A shows an alternative embodiment of the invention suitable for correcting raster barrel distortion.

FIG. 4B shows an alternative embodiment of the invention suitable for connecting raster pincushion distortion.

The preferred geometries of the embodiments of FIGS. 4A and 4B are discussed hereinafter.

DESCRIPTION OF OPERATION As discussed hereinbefore, in a flat cathode ray tube such as that shown in FIGS. 11 and 2, the electron beam when deflected to the far end of screen 32 intersects the screen at an acute angle 40. If that beam has a circular cross-section normal to its path, it produces an elliptical (or elongated) spot on the screen. Even if the beam has an elliptical cross-section normal to the direction of elongation, some widening of the region of intersection of the beam with the screen may occur, especially in those regions of the screen most remote from the center of deflection, where the angle of incidence between beam and screen is smallest. As aforementioned, elongated spots are undesirable because they cover unnecessarily large areas of the screen and hence defocus the image produced, thereby degrading the image resolution obtainable with the tube.

In accordance with the invention, this defocusing effeet is minimized by causing the electron beam, when deflected to the far end of the tube, to be bent as shown by broken line 42 so that it intersects screen 32 at an angle 44 closer to Bending of the beam in this manner is accomplished by applying a varying electrostatic force to the electrons of the beam, as they move from deflection apparatus 8 toward screen 32.

The force exerted on an electron in an electric field between two plane surfaces is directly proportional to the difference in potential applied to the surfaces and inversely proportional to the distance between the surfaces, and is exerted in the direction of more positive potential.

In the embodiment of the invention shown in FIGS. 2 and 3, a potential of about 15,000 volts is applied both to screen 32, i.e. aluminum layer 35, and to conductive section 26, a potential of about 12,500 volts is applied to conductive section 22 and a potential of about 10,000 volts is applied to conductive section 18. Thus in the near region between layer 35 and conductive section 26, since both layer 35 and section 26 are at the same potential, no electrostatic field exists therebetween. Accordingly when the beam is deflected along path 33 to the near side of screen 32 by deflection means 8, the beam remains entirely in this zero electrostatic field region; hence, no bending force is exerted upon it. However, when the beam is deflected toward the center of screen 32 along path 34 it must pass through a portion of the central region between screen 32 and conductive section 22. Because aluminum layer 35 is biased substantially positive, e.g. about 2500 volts, with respect to conductive section 22, an electrostatic field exists therebetween, which forces the electrons toward screen 32. Thus the beam initially passes through a near region in which no bending force is exerted and then passes through a control region in which a moderate bending force is exerted.

An even larger bending force is exerted on the beam in the far region bounded by conductive section 18 and layer 35 because of the even smaller distance and the even larger potential difference of about 5,000 volts therebetween. Because the backplate 14 is angled slightly with respect to the frontplate, being closer to the frontplate 4 at the far end 3 of the tube, there also is a slight increase in bending force within each region in the direction toward end 3. Thus when the beam 42 is deflected toward the far end of screen 32 it first passes through a near region of zero bending, then through a control region of moderate bending which increases as the beam approaches closer to end 3, and finally through a far region of more severe bending which also increases as the beam approaches closer to end 3.

Although in the embodiment of the invention described herein, operating potentials of about 15,000 volts, about 12,500 volts, and about 10,000 volts are used, different operating potentials may be employed, particularly where the tube size or geometry is varied from that specifically illustrated. The operation of the aforedescribed electrostatic beam bending system depends in significant part on the relative magnitudes of the respective voltages supplied to conductive sections 18, 22 and 26. Preferably the voltage supplied to conductive section 18 is between about one-third and about seven-eighths of the voltage supplied to conductive section 26, and the voltage supplied to conductive section 22 is approximately half the sum of the respective voltages supplied to conductive sections 18 and 26. The exact relationship chosen will depend on the specific geometry of the tube. When the voltage supplied to conductive section 22 is made smaller relative to the voltage supplied to screen 32 the bending force in the central region becomes larger, the beam curves toward the screen 32 sooner, and a beam directed at the center of screen 32 impinges closer to the near side of the tube than would occur in the absence of the bending field. Any inaccuracy in the raster introduced by such bending can be compensated by deflecting the beam by an appropriate additional amount toward the far side of screen 32. However, when the voltage supplied to conductive region 22 is made too low, the additional compensation required to make the beam impinge at the correct position on screen 32 will create a neck-shadow problem. That is, in order to position a beam at the far end of screen 32 the beam will have to be deflected upwardly to such an extent that the beam will strike the edge of neck section 10 and never reach screen 32. Moreover, when the voltage on conductive section 22 is made too high, insufficient bending will occur in the center region of the tube and the resultant image produced on screen 32 will become more defocused in the center than at the edges.

Table 1 illustrates the improvement in focussing obtained by the present invention in a flat cathode ray tube, having screen dimensions of approximately 10 inches by 7.5 inches, and employing an electron gun having elliptical apertures, the major axis of each of which is twice as long as its minor axis.

Table I Effect of Electrostatic Field on Spot Size Potential (with respect to cathode potential) applied to focus electrode of electron gun is 1100 volts volts.

Operating Potential Relative Spot Size Sec. Sec. Sec. Dimen- Near Re- Center Far 18 22 26 sion gion Region Region (neck end) (No Graded Field) width 1.2 3

4.8 A 15 kv i5 kv 15 kv height 1.0 1.8

2.7 width 1.2 2.7

1.3 B 10 kv 13.5 kv l5 kv height 1.0 1.8

2 width 1.2 2.5

1.2 C 10 kv 12.5 kv l5 kv height 1.0 1.8

Entry A in Table I shows the relative width and height of the spot made by the electron beam impinging on the screen in each of the three regions with no beam bending correction applied (i.e., the same voltage on all three conductive sections). The spot sizes shown are normalized to the minor axis of the beam when it impinges the screen in the near region of the tube. Entries B and C show the improvement in beam focusing obtained by practice of the invention. It is seen that focussing is substantially improved in the center region of the tube and very significantly improved in the far region of the tube.

Raster distortion effects in flat cathode ray tubes are most pronounced at the end of the screen farthest from the neck section. This occurs because the angle between the path of the beam at its point of impingement with the screen 32 is smallest at the far end of the screen. Thus in a flat cathode ray tube having conductive sections 18, 22 and 26 of the form shown in FIGS. 2 and 3, raster distortion effects will be most pronounced at the edge of screen 32 located nearest end 3 of the tube.

According to the invention, raster distortion is minimized by properly shaping the conductive sections at the far end of the tube as shown in FIGS. 4A and 4B. The embodiment of the invention shown in FIG. 4A reduces barrel-type raster distortion. When barrel-type raster distortion occurs, the edge of the raster takes on a convex shape (i.e. bowed out in the center portion of the raster) as shown by line 46. It will be appreciated from the previous description, that by shaping conductive sections 18a and 22a as shown in FIG. 4A, a smaller bending force will be applied to the electron beam when deflected toward the far edge of screen 32 along path 48 or along path 52 than when deflected along path 50. In this manner the corners of the screen become filled in and barrel distortion is significantly reduced.

Curved edges of conductive sections and 22a having radii of curvature of between about one-half and about twice the width of screen 32 (i.e. the length of the edge of screen 32 illustrated in FIG. 2) have been found in practice to produce good results. If those radii of curvature are too large there is insufficient barrel correction and if those radii of curvature are too small the neck shadow problem, hereinbefore described, arises for those beams deflected along path 50 (see FIG. 4A). in addition, where the radii of curvature are too small, there is excessive variation of spot size along the far edge of the screen.

The embodiment of the invention shown in FIG. 4B corrects for pincushion distortion (illustrated by dashed line 54). The principle of operation for pincushion correction is similar to that hereinbefore described for barrel correction, except that the electrostatic field is shaped to provide less bending for those beams deflected along the center of the screen than for those beams deflected toward the corners of the screen.

Although specific embodiments of the invention for correction of barrel and pincushion distortion have been given, it will be apparent to those skilled in the art that the invention can also be adapted by suitable shaping of the conductive sections to correct for other types of raster distortion (e.g. keystone" distortion) and to correct for both vertical and horizontal nonlinearity. Moreover, although the conductive sections 18 and 22 illustrated in FIG. 4A are shown as comprising arcuate edges, edges made up of straight-line-segments, e.g. like those shown in FIG. 413, may be substituted for those arcuate edges. However when so substituted such straight-line-segments should slant away from edge 3, instead of toward edge 3 as in FIG. 413. Similarly, in the embodiment of FIG. 4B, arcuate edges convex toward edge 3 may be substituted for the straight-line-segment edges illustrated in FIG. 4B. Where straight-line-segment edges are employed, they need not be made up of only two line-segments but alternatively may comprise three or more line-segments. In addition, the invention applies equally as well to other beam deflection devices such as electron microscopes, storage tubes, or radar and communications displays.

We claim:

ll. In a cathode ray tube comprising a substantially rectangular backplate, a substantially flat rectangular electron beam responsive screen opposing said backplate, separated therefrom, and tilted so that a first edge of said screen is closed to said backplate than is a second edge of said screen opposite said first edge, and electron gun means for projecting a beam of electrons against said screen, said electron gun means being positioned (i) outside the region bounded by said screen and said backplate and (ii) nearer to said second edge of said screen than to said first edge thereof, and being oriented so that, when no deflection force is applied to said beam of electrons projected thereby, said beam strikes said screen at an acute angle, said tube being adapted to have said beam of electrons deflected about a given deflection center and being constructed so that a line connecting said deflection center and the center of said screen forms an acute angle with said screen,

the improvement comprising:

first, second and third discrete conductive coatings respectively positioned on first, second and third discrete portions of said backplate, said first discrete portion being the farthest of said three discrete portions from said screen, said third discrete portion being the nearest of said three discrete portions to said screen, and said second discrete portion being between said first discrete portion and said third discrete portion, said three conductive coatings being the only conductive coatings on said backplate, and

means for applying a constant potential to each of said three conductive coatings. 2. A system comprising the tu e of claim 1 and also comprising means for deflecting said beam over said screen in respectively transverse directions, and a source for supplying a first constant potential to said first conductive coating and said screen, a second constant potential more negative than said first constant potential to said second conductive coating, and a third constant potential more negative than said second constant potential to said third conductive coating, only constant potentials being supplied to said conductive coatings.

3. The system of claim 2 wherein said means for deflecting comprises yoke means for producing mutually orthogonal magnetic deflection fields in response to electric currents supplied thereto.

4. The system of claim 3 wherein the magnitude of said third constant potential is between about one-third and about seven-eighths of the magnitude of said first constant potential and the magnitude of said second constant potential is about one-half the sum of the magnitudes of said first and third potentials.

5. The tube of claim 1 wherein the edge of said first conductive coating nearest said first edge of said screen is substantially parallel thereto, and each of said second and third conductive coatings is substantially rectangular, with edges substantially parallel to said edge of said first conductive coating.

6. The tube of claim 1 wherein the edge of said first conductive coating nearest said first edge of said screen is substantially parallel thereto, the edge of said second conductive coating adjacent said edge of said first conductive coating is substantially parallel to the latter edge, and the adjacent edges of said second and third conductive coatings are curved.

7. The tube of claim 6 wherein each of said curved edges of said second conductive coating and said third conductive coating conforms to an are which is substantially symmetrical with respect to that center line of said backplate transverse to said edge of said first conductive coating nearest said first edge of said screen, has a radius of curvature of between about one-half and about twice the width of said screen, and is nearest said edge of said first conductive coating at its intersection with said center line.

8. The tube of claim 6 wherein the adjacent edges of said second and third conductive coatings are farthest from said edge of said first conductive coating nearest said first edge of said screen, at the respective intersections of said adjacent edges with that center line of said backplate transverse to said edge of said first conductive coating. 

1. In a cathode ray tube comprising a substantially rectangular backplate, a substantially flat rectangular electron beam responsIve screen opposing said backplate, separated therefrom, and tilted so that a first edge of said screen is closed to said backplate than is a second edge of said screen opposite said first edge, and electron gun means for projecting a beam of electrons against said screen, said electron gun means being positioned (i) outside the region bounded by said screen and said backplate and (ii) nearer to said second edge of said screen than to said first edge thereof, and being oriented so that, when no deflection force is applied to said beam of electrons projected thereby, said beam strikes said screen at an acute angle, said tube being adapted to have said beam of electrons deflected about a given deflection center and being constructed so that a line connecting said deflection center and the center of said screen forms an acute angle with said screen, the improvement comprising: first, second and third discrete conductive coatings respectively positioned on first, second and third discrete portions of said backplate, said first discrete portion being the farthest of said three discrete portions from said screen, said third discrete portion being the nearest of said three discrete portions to said screen, and said second discrete portion being between said first discrete portion and said third discrete portion, said three conductive coatings being the only conductive coatings on said backplate, and means for applying a constant potential to each of said three conductive coatings.
 2. A system comprising the tube of claim 1 and also comprising means for deflecting said beam over said screen in respectively transverse directions, and a source for supplying a first constant potential to said first conductive coating and said screen, a second constant potential more negative than said first constant potential to said second conductive coating, and a third constant potential more negative than said second constant potential to said third conductive coating, only constant potentials being supplied to said conductive coatings.
 3. The system of claim 2 wherein said means for deflecting comprises yoke means for producing mutually orthogonal magnetic deflection fields in response to electric currents supplied thereto.
 4. The system of claim 3 wherein the magnitude of said third constant potential is between about one-third and about seven-eighths of the magnitude of said first constant potential and the magnitude of said second constant potential is about one-half the sum of the magnitudes of said first and third potentials.
 5. The tube of claim 1 wherein the edge of said first conductive coating nearest said first edge of said screen is substantially parallel thereto, and each of said second and third conductive coatings is substantially rectangular, with edges substantially parallel to said edge of said first conductive coating.
 6. The tube of claim 1 wherein the edge of said first conductive coating nearest said first edge of said screen is substantially parallel thereto, the edge of said second conductive coating adjacent said edge of said first conductive coating is substantially parallel to the latter edge, and the adjacent edges of said second and third conductive coatings are curved.
 7. The tube of claim 6 wherein each of said curved edges of said second conductive coating and said third conductive coating conforms to an arc which is substantially symmetrical with respect to that center line of said backplate transverse to said edge of said first conductive coating nearest said first edge of said screen, has a radius of curvature of between about one-half and about twice the width of said screen, and is nearest said edge of said first conductive coating at its intersection with said center line.
 8. The tube of claim 6 wherein the adjacent edges of said second and third conductive coatings are farthest from said edge of said first conductive coating nearest said first edge of said screen, at the respective intersections of said adjacent edges with tHat center line of said backplate transverse to said edge of said first conductive coating. 